This is the classical Four Knights' Game. One of the beginning principles of the opening that the beginners learn is "Knights before bishops", and thus dogmatic followers of this principle stick to this system. All four knights have been developed (hence the name), leading to a symmetrical position. The classical players like Maroczy, Capablanca, Lasker, Tarrasch, and Rubinstein would play the Symmetrical Variation, 4. Bb5 Bb4 5. O-O O-O 6. d3 d6 7. Bg5, when Black should sever the symmetry with 7...Bxc3 (if 7...Bg4, then 8. Bxf6 Qxf6? (8...gxf6 is better, but leaves White with the better game because of Black's weakened kingside) 9. Nd5 Qd8 10. Bxc6 bxc6 11. Nxb4, when White is up a piece) 8. bxc3, with an even game. However, Rubinstein's 4...Nd4 and Marshall's 4...Bc5 are sharp alternatives. The Scotch Four Knights begins with 4.d4, and can lead to the enterprising Belgrade Gambit after 4...exd4 5.Nd5!?, a highly tactical system which often runs 4...exd4 5. Nd5 Nxe4 6. Bc4 Be7 7. Nxd4 O-O 8. Nb5 Bc5 9. O-O Nxf2 11. Qh5. Black is doing all right, but some careful play is required.